Fix.

node_modules/diff-sequences/perf/example.md

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+## Benchmark time for `diff-sequences` versus `diff`
+
+A ratio less than 1.0 means `diff-sequences` is faster.
+
+### n = 20
+
+| name   |    % | ratio  | improved  |   rme | baseline  |   rme |
+| :----- | ---: | :----- | :-------- | ----: | :-------- | ----: |
+| delete |  20% | 0.1824 | 3.0639e-6 | 1.11% | 1.6795e-5 | 0.78% |
+| insert |  20% | 0.1367 | 2.5786e-6 | 0.75% | 1.8866e-5 | 0.85% |
+| delete |  40% | 0.1015 | 3.0534e-6 | 1.00% | 3.0090e-5 | 0.70% |
+| insert |  40% | 0.0791 | 2.6722e-6 | 0.61% | 3.3791e-5 | 0.56% |
+| delete |  80% | 0.0410 | 2.8870e-6 | 0.93% | 7.0411e-5 | 0.72% |
+| insert |  80% | 0.0371 | 2.5786e-6 | 0.83% | 6.9431e-5 | 0.62% |
+| change |  10% | 0.1504 | 2.8703e-6 | 0.71% | 1.9087e-5 | 0.83% |
+| change |  20% | 0.1706 | 3.2637e-6 | 0.78% | 1.9127e-5 | 0.62% |
+| change |  50% | 0.0944 | 1.2012e-5 | 0.55% | 1.2724e-4 | 0.76% |
+| change | 100% | 0.0522 | 1.5422e-5 | 0.61% | 2.9566e-4 | 0.66% |
+
+### n = 200
+
+| name   |    % | ratio  | improved  |   rme | baseline  |   rme |
+| :----- | ---: | :----- | :-------- | ----: | :-------- | ----: |
+| delete |  20% | 0.1524 | 7.2866e-5 | 0.75% | 4.7797e-4 | 0.80% |
+| insert |  20% | 0.1226 | 6.1561e-5 | 0.58% | 5.0198e-4 | 0.66% |
+| delete |  40% | 0.1118 | 1.5674e-4 | 0.67% | 1.4020e-3 | 0.58% |
+| insert |  40% | 0.0894 | 1.2906e-4 | 0.64% | 1.4435e-3 | 0.53% |
+| delete |  80% | 0.0796 | 3.0119e-4 | 0.58% | 3.7852e-3 | 0.52% |
+| insert |  80% | 0.0734 | 2.4713e-4 | 0.67% | 3.3653e-3 | 0.54% |
+| change |  10% | 0.1572 | 7.2965e-5 | 0.48% | 4.6426e-4 | 0.73% |
+| change |  20% | 0.1446 | 7.0056e-5 | 0.69% | 4.8456e-4 | 0.53% |
+| change |  50% | 0.0764 | 6.5638e-4 | 0.67% | 8.5946e-3 | 0.70% |
+| change | 100% | 0.0525 | 1.1160e-3 | 0.51% | 2.1249e-2 | 0.63% |
+
+### n = 2000
+
+| name   |    % | ratio  | improved  |   rme | baseline  |   rme |
+| :----- | ---: | :----- | :-------- | ----: | :-------- | ----: |
+| delete |  20% | 0.0669 | 3.4073e-3 | 0.54% | 5.0922e-2 | 0.54% |
+| insert |  20% | 0.0588 | 2.8273e-3 | 0.51% | 4.8111e-2 | 0.46% |
+| delete |  40% | 0.0517 | 1.1048e-2 | 0.52% | 2.1367e-1 | 0.47% |
+| insert |  40% | 0.0460 | 9.1469e-3 | 0.37% | 1.9878e-1 | 0.26% |
+| delete |  80% | 0.0563 | 2.7426e-2 | 0.56% | 4.8674e-1 | 0.36% |
+| insert |  80% | 0.0506 | 2.2208e-2 | 0.35% | 4.3888e-1 | 0.47% |
+| change |  10% | 0.0716 | 3.1267e-3 | 1.21% | 4.3652e-2 | 0.56% |
+| change |  20% | 0.0621 | 3.0197e-3 | 0.72% | 4.8652e-2 | 0.45% |
+| change |  50% | 0.0083 | 5.4250e-2 | 0.62% | 6.5595e+0 | 3.60% |
+| change | 100% | 0.0493 | 1.0534e-1 | 0.71% | 2.1362e+0 | 0.21% |

node_modules/diff-sequences/perf/index.js

@@ -0,0 +1,155 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// Make sure to run node with --expose-gc option!
+
+// The times are reliable if about 1% relative mean error if you run it:
+
+// * immediately after restart
+// * with 100% battery charge
+// * not connected to network
+
+/* eslint import/no-extraneous-dependencies: "off" */
+
+const Benchmark = require('benchmark');
+
+const diffBaseline = require('diff').diffLines;
+const diffImproved = require('../build/index.js').default;
+
+const testBaseline = (a, b) => {
+  const benchmark = new Benchmark({
+    fn() {
+      diffBaseline(a, b);
+    },
+    name: 'baseline',
+    onCycle() {
+      global.gc(); // after run cycle
+    },
+    onStart() {
+      global.gc(); // when benchmark starts
+    },
+  });
+
+  benchmark.run({async: false});
+
+  return benchmark.stats;
+};
+
+const testImproved = function(a, b) {
+  const benchmark = new Benchmark({
+    fn() {
+      // Split string arguments to make fair comparison with baseline.
+      const aItems = a.split('\n');
+      const bItems = b.split('\n');
+
+      const isCommon = (aIndex, bIndex) => aItems[aIndex] === bItems[bIndex];
+
+      // This callback obviously does less than baseline `diff` package,
+      // but avoiding double work and memory churn is the goal.
+      // For example, `jest-diff` has had to split strings that `diff` joins.
+      const foundSubsequence = () => {};
+
+      diffImproved(aItems.length, bItems.length, isCommon, foundSubsequence);
+    },
+    name: 'improved',
+    onCycle() {
+      global.gc(); // after run cycle
+    },
+    onStart() {
+      global.gc(); // when benchmark starts
+    },
+  });
+
+  benchmark.run({async: false});
+
+  return benchmark.stats;
+};
+
+const writeHeading2 = () => {
+  console.log('## Benchmark time for `diff-sequences` versus `diff`\n');
+  console.log('A ratio less than 1.0 means `diff-sequences` is faster.');
+};
+
+const writeHeading3 = n => {
+  console.log(`\n### n = ${n}\n`);
+  console.log('| name | % | ratio | improved | rme | baseline | rme |');
+  console.log('| :--- | ---: | :--- | :--- | ---: | :--- | ---: |');
+};
+
+const writeRow = (name, percent, statsImproved, statsBaseline) => {
+  const {mean: meanImproved, rme: rmeImproved} = statsImproved;
+  const {mean: meanBaseline, rme: rmeBaseline} = statsBaseline;
+  const ratio = meanImproved / meanBaseline;
+
+  console.log(
+    `| ${name} | ${percent}% | ${ratio.toFixed(
+      4,
+    )} | ${meanImproved.toExponential(4)} | ${rmeImproved.toFixed(
+      2,
+    )}% | ${meanBaseline.toExponential(4)} | ${rmeBaseline.toFixed(2)}% |`,
+  );
+};
+
+const testDeleteInsert = (tenths, more, less) => {
+  // For improved `diff-sequences` package, delete is often slower than insert.
+  const statsDeleteImproved = testImproved(more, less);
+  const statsDeleteBaseline = testBaseline(more, less);
+  writeRow('delete', tenths * 10, statsDeleteImproved, statsDeleteBaseline);
+
+  // For baseline `diff` package, many insertions is serious perf problem.
+  // However, the benchmark package cannot accurately measure for large n.
+  const statsInsertBaseline = testBaseline(less, more);
+  const statsInsertImproved = testImproved(less, more);
+  writeRow('insert', tenths * 10, statsInsertImproved, statsInsertBaseline);
+};
+
+const testChange = (tenths, expected, received) => {
+  const statsImproved = testImproved(expected, received);
+  const statsBaseline = testBaseline(expected, received);
+  writeRow('change', tenths * 10, statsImproved, statsBaseline);
+};
+
+const getItems = (n, callback) => {
+  const items = [];
+
+  for (let i = 0; i !== n; i += 1) {
+    const item = callback(i);
+    if (typeof item === 'string') {
+      items.push(item);
+    }
+  }
+
+  return items.join('\n');
+};
+
+// Simulate change of property name which is usually not same line.
+// Expected: 0 1 2 3 4 5 6 7 8 9 and so on
+// Received: 1 2 3 4 x0 5 6 7 8 9 and so on
+const change2 = i => {
+  const j = i % 10;
+  return j === 4 ? `x${i - 4}` : j < 4 ? `${i + 1}` : `${i}`;
+};
+
+const testLength = n => {
+  const all = getItems(n, i => `${i}`);
+
+  writeHeading3(n);
+
+  [2, 4, 8].forEach(tenth => {
+    testDeleteInsert(tenth, all, getItems(n, i => i % 10 >= tenth && `${i}`));
+  });
+  testChange(1, all, getItems(n, i => (i % 10 === 0 ? `x${i}` : `${i}`)));
+  testChange(2, all, getItems(n, change2));
+  testChange(5, all, getItems(n, i => (i % 2 === 0 ? `x${i}` : `${i}`)));
+  testChange(10, all, getItems(n, i => `x${i}`)); // simulate TDD
+};
+
+writeHeading2();
+
+testLength(20);
+testLength(200);
+testLength(2000);